For the marking and thus individual correlation and verification of check cards, credit cards, access cards, electronic keys or the like, a number of magnetic codings are used, and mainly in the form of a so-called magnetic strip. For the coding a permanent magnetic layer is selectively, i.e. regionally, so magnetized that regions of different magnetization result. In the sense of the present description nonmagnetized regions, i.e. regions of zero magnetization, are included as regions of different magnetization. With corresponding field sensors, the magnetic signature or coding can be determined and processing for the respective purpose can be carried out.
There are numerous proposals for the formation of magnetic strips. In U.S. Pat. No. 4,650,978, the naturally occurring random variations of the magnetic characteristics of the magnetic strip, having their origins in the variations in coercivity, granularity, layer thickness, surface contour and especially in random variations in the hysteresis loop and other magnetic histories determining same are used. Similar are the codings in the methods according to U.S. Pat. No. 5,616,904 and U.S. Pat. No. 4,837,826. The magnetic structure is thus digitized in a suitable form and used to identify the object. It is a drawback that the magnetic strips can be manipulated relatively simply and are not resistant to external magnetic fields.
In U.S. Pat. Nos. 5,480,658 and 5,972,438 magnetic strips are described in which magnetic particles are incorporated in a binder, whereby the magnetic strips each have two layers of different coercivities. Also the magnetic strips of U.S. Pat. No. 5,177,344 have magnetic particles in a binder matrix whereby the magnetic particles are so influenced by the application of an external magnetic field that they yield magnetic regions of different characteristics. These types of magnetic strips have the drawback that the magnetic structure can be subsequently altered by heating of the binder and by newly orienting the magnetic particles by an external magnetic field. In the magnetic strips of U.S. Pat. No. 5,365,586 a number of microcrystalline structures are arranged in a random grid. The magnetic strips are thus subjected to saturation magnetization, whereby the remanent noise is read out and used for identification. These magnetic strips as well are capable of being influenced by external manipulations.
In U.S. Pat. No. 5,254,843, conventionally written magnetic bands and strips have random variations in time sequence of flux changes which are used for identification of the respective object. Here as well the random structure, upon identification, can easily be reproduced. Aside from this, the method requires a randomness of the variations in time which, especially in the case of machine writing, cannot be taken for granted.
In the PCT/EP99/08433, which was not prepublished, a marking device is proposed in which the coding has a magnetic base layer and a magnetic coding layer which so cooperate that over the extent of the base layer and the coding layer, there are regions with nonparallel or antiparallel magnetic coupling. In this case use is made of the effect of magnetic interlayer coupling. The marking device has the advantage of a highly characteristic property distinguishing from the usual magnetic marking that upon the application of an external magnetic field while the nonparallel or antiparallel coupling is broken by the influence of a saturation magnetic field, the original magnetization is restored after the removal of the external magnetic field. The coding can thus not be extinguished by external magnetic fields. In addition the effect can be used, for example, in the case of magnetic codings which weaken and are even lost as a result of long storage time, to reactivate them by exposing them to a saturation magnetic field.